WASHINGTON—The International Committee of the Red Cross said Thursday that waterboarding is torture, the first time the group has publicly declared that a specific interrogation technique—one employed by the U.S.—violates the Geneva Conventions.

The Geneva-based humanitarian organization, which oversees the treaty, weighed in amid renewed U.S. debate over Central Intelligence Agency practices after the Sept. 11, 2001, attacks, which included waterboarding several detainees at so-called black sites overseas. The CIA and...